PART I Historical

they ate the oxen of the sun,
the herd of HeIios Hyperion
Homer, The Odyssey
(translation by Mandelbaum)

The history of understanding structure in our Universe is older than
the story of Odysseus, and has as many twists and turns. Few of these
paths remain familiar to most astronomers today, so in the early
chapters I have simply collected some essential developments along the
way. They are not without surprises. One of which is that many ideas
now thought to be novel have really been known for tens, or hundreds,
of years. Often they were no more than speculations, but sometimes
they captured reality's core.

To every thing there is a season,
and a time to every purpose under heaven.
Ecclesiastes 3:1

Structure may be the most surprising property of our Universe.
Structure differentiates things, and the changing of things introduces
the notion of time. We can easily imagine universes without
macroscopic structure, filled by an equilibrium distribution of matter
and radiation: homogeneous, isotropic, and dull. Indeed such a state
may have described our own Universe until fairly recently. The oldest
structures we know are the high-redshift galaxies and quasars, dating
from an epoch when the Universe was only about ten times smaller than
it is now. Compare this with the epoch of decoupling of matter and
radiation when the scale of the Universe was a thousand times smaller
than now, with the epoch of nucleosynthesis at which helium formed
when the Universe was a billion times smaller, and with the almost
mythical quantum epoch when the Universe was more than 1050 times
smaller.

Although the structure we now see may be considered fairly
recent when viewed in terms of the expanding scale of the Universe, it
is very old in terms of total time elapsed since the expansion
began. This is because the relative rate of expansion of the scale of
the Universe, (t) /
R(t), slows down as the Universe expands. In
typical conventional cosmological models, the distance of the farthest
galaxies and quasars corresponds to more than 90% of the age of the
Universe since the big bang. Earlier structure no doubt existed, but
our knowledge of it is much more uncertain.

Long before people knew the age and size of any astronomical
structure, civilizations imagined many origins for the Earth and the
Moon, the Sun and the stars. These cosmogony myths varied widely
among different cultures, and sometimes rival myths arose within the
same culture. Most of these myths begin with a magnified version of
the locally familiar - rocks, oceans, personified gods with human
vices, a giant clam, even a mirror world beyond the sky. Such basic
elements, like some aspects of modern cosmology, had no origin
themselves and were subject to no further questioning. Eventually
there occurred a split, disruption, battle, or sexual union from which
the Earth, Moon, Sun, oceans, sky. remaining gods, first people,
plants, and animals emerge in assorted ways and combinations for each
different myth. In several myths a cosmic egg forms or is produced.
Something breaks it, or it opens spontaneously, and the structure of
the universe emerges. Putting everything in its place, however, may
require further work by the gods.

Cosmogony myths are usually grouped into historical or
anthropological categories. But here, to emphasize their range, I
suggest a thematic classification whose sequence tends toward
increasing abstraction. Some examples represent each type and
illustrate how many types contain common features. Naturally we view
these myths and early cosmogonies through the lens of modern
understanding. Any claim to decant the detailed attitudes of their
original believers from our own concepts of the Universe would be
largely illusory.

In the first group of myths, battles among the gods generate
structure. Among the oldest is the Babylonian Epic of the Creation
whose earliest known written tablets go back three thousand years and
are probably based on still older versions. Not surprisingly, the
Universe was organized by the god of Babylon himself, Marduk. He was
the grandson of Apsu and Tiamet who personified the sweet and salt
waters, from whose mingling was born a family of gods. They squabbled
long and noisily, giving each other insomnia and indigestion, until
Apsu and Tiamet disagreed over whether they should destroy their own
progeny. Battle lines were drawn. Marduk, potentially the bravest,
was selected king of the gods. He killed Tiamet by commanding the
tempests and hurricanes to blow her full of wind until she was so
puffed up she could not close her mouth. Then he shot an arrow into
her heart and split her body into two halves, one forming the heavens
and the other forming the Earth. After that it was relatively easy to
organize the world, install the stars, fix the planetary orbits, and
create humanity to do the hard work and leave the clods free to amuse
themselves.

The old Greek version of the separation of Heaven and Earth is
told by Hesoid (ca. 800 B.c.; see also Lang, 1884). His Theogony, an
extensive geneology of three hundred gods, may derive its style partly
from the earlier Babylonian epic, although this is unclear. It starts
with Chaos, not in the sense of turbulence and erratic unpredictable
motion we use today, but in the earlier sense of a vast formless dark
chasm or void. Chaos was not a god, but a sort of principle, which
produced the Earth (Gaia), underworld (Tartara), and love (Eros)
gods. Earth bore Heaven (Uranus), then together they produced a bunch
of nasty children, including Kronos whom Gaia encouraged to castrate
Uranus. Thus Earth and Heaven were separated and a slew of other gods,
nymphs, and giants developed from the bits and pieces of the wound.

Alternatively, according to the myths of Iceland. ice played a
major role in the formation of the world from the void. One part of
the void developed a region of clouds and shadows, another a region of
fire, a third a fountain from which flowed rivers of ice. As some of
this ice melted, Ymir, a giant who looked like a man, emerged, and
then a giant cow to feed him along with other giants and giant men and
women. These bore three gods who battled the giants, killing Ymir. His
flesh became the land, his blood the sea, his bones the mountains, his
hair the trees, and his empty skull the sky. Stray sparks from the
region of the fire were placed in his skull by the gods to form the
Moon, Sun, and stars, which were then set in motion. Maggots forming
in Ymir's rotting corpse were made into the dwarfs of the
underworld. Eventually the gods made humans from tree trunks.

A more abstract battle cosmogony was developed by the ancient
Persians, especially the Zoroastrians. Two personifications, good
(Ormazd) and evil (Ahriman), alternately created the opposing aspects
of the world: light - darkness, life - death, truth - falsehood,
summer - winter, pretty birds - biting insects, and so on for three
thousand years. After another six thousand years Ormazd would win and
the world would be purified. Observations can therefore put an upper
limit on the age of this cosmogony.

In the second group of myths, the gods generate structure much
more peacefully. In Genesis this is done by monotheistic decree
rather than by magnifying tribal or family strife. First, light is
created in the dark void, then the heaven and Earth are separated and
heaven supplied with the Sun, Moon, and stars. In contrast, Japanese
creation myths (perhaps partly derived from Chinese sources) began
with three gods forming spontaneously in heaven who then hid
themselves away. The Earth, which was first like a huge globule of oil
floating on a vast sea, gave rise to two further hiding gods, and then
seven generations of more normal gods. Izanagi and his wife
Izanami. the last of these generations, caused the Earth to solidify
by poking it with a spear given to them by other gods. They stood on
the floating bridge of heaven during this process and a drop falling
as they lifted up the spear formed the island of Onokoro, on which
they landed. Their subsequent union gave birth to the rest of the
Japanese islands (after some misbirths caused by the woman speaking
first to the man, rather than waiting until she was spoken to) and
then to a new range of nature gods. Izanami died in childbirth when
the fire god was born and went to hell, where the distraught Izanagi
followed trying to convince her to return. He failed, and after
several misadventures returned to the surface where he cleansed
himself by bathing in the sea and producing more gods. The greatest of
these was the Sun goddess, Amaterasu, who resulted from Izanagi
washing his left eye; the moon goddess, Tsukiyomi, came from washing
his right eye. Thus were the Earth, Sun, and Moon created without the
need for great battles.

The Pawnee Indians of Nebraska also had a peaceful cosmogony
generated by decree. Their chief god, Tirawa, first placed the Sun in
the east, and the Moon and the evening star in the west, the pole star
in the north, and the Star of Death in the south. Four other stars
between each of these supported the sky. He assembled the lightning,
thunder, clouds, and winds creating a great dark storm. Dropping a
pebble he caused the thick clouds to open revealing vast waters. Then
Tirawa ordered the four star gods holding up heaven to smite the
waters with maces, and lo the waters parted and Earth became
manifest. Next he ordered the four star gods to sing and praise the
creation of Earth, whence another huge storm rose up gouging out
mountains and valleys. Three times more the gods sang and there came
forth forests and prairies, flowing rivers, and seeds that grow. The
first human chief was born to the Sun and the Moon, the first woman to
the morning and evening star. The rest of humanity was created by the
union of stars.

The third group of myths are less anthropomorphic animal
cosmogonies. Islanders on Nauru, in the South Pacific, say that in the
beginning only an Old-Spider floated above the primordial sea. One day
she discovered a giant clam and looked in vain for a hole to enter
by. So she tapped the clam. It sounded hollow and empty. With the aid
of a magic incantation she opened the shell a bit and crawled inside
where it was cramped and dark. Eventually, after a long search
throughout the clam, Old-Spider found a snail. Although the snail was
useless in its original form, she give it great power by sleeping with
it under one of her arms for three days. Releasing the snail, she
explored some more and found another, bigger snail, which got the same
treatment. Politely, Old-Spider asked the newly energized first snail
to open the clam a little wider to give them more room. As its reward,
she turned the snail into the Moon. By the feeble light of the Moon,
Old-Spider next found an enormous worm. Stronger than the snail, the
worm raised the shell a little higher as the salty sweat poured from
its body to form the sea. Higher still and higher he raised the upper
half-shell until it became the sky. But the effort was too much, and
the worm died of exhaustion. Old-Spider turned the second snail into
the Sun near the lower half-shell, which formed the Earth.

Egg cosmogonies are the fourth group of myths. Present in the
early Egyptian and Phoenician creation stories, they became especially
prominent in the Greek Orphic and Indian Vedic cosmogonies. Followers
of Orpheus believed that Ether, the spirit of the finite, gradually
organized the cosmic matter into an enormous egg. Its shell was night,
its upper part was the sky, and its lower part the Earth. Light, the
first being, was born in its center and, combining with Night, created
Heaven and Earth. In the Vedic cosmogonies, a golden egg arose and the
Universe and all its contents were nascent within. After a thousand
years the egg opened and Brahma emerged to begin orga- nizing the
Universe, first becoming a wild boar to raise the Earth above the
waters.

A Finno-Ugric multi-egg cosmogony begins with the Daughter of
Nature, Luonnotar, who grew bored of floating through the sky and
descended to the sea where she lay amidst the waves for seven
centuries. A large bird wanting to build a nest spied her knee
sticking out of the sea. So the bird laid eggs there and sat on them
for three days, until Luonnotar became irritated and flung them
off. When they cracked open their lower parts combined to form the
Earth, their upper parts became heaven, their yolks became the Sun,
their whites the Moon, and the spotted bits formed the stars.

The fifth group of myths have inanimate origins. Sometimes, as
with the primordial Egyptian concept of Nun it is the ocean that gives
rise even to the gods. Elsewhere, as in Samoa, the Universe began as
a series of rocks, first the rocks of heaven and then those of
Earth. The Hawaiians had a myth in which the Universe was cyclic, each
world emerging from the wreck of a previous one.

The sixth, most abstract, group of myths do not involve gods
at all. Structure in these universes is completely self-generating. On
the Marquesas Islands in the South Pacific is a myth that describes
how "the primeval void started a swelling, a whirling, a vague growth,
a boiling, a swallowing; there came out an infinite number of supports
or posts, the big and the little, the long and the short, the hooked
and the curved, and above all there emerged the solid Foundation,
space and light, and innumerable rocks" (Luquet, 1959). This is not
far from some modern cosmologies if we substitute "vacuum" for "void,"
"perturbations" for "supports or posts," and "galaxies" for "rocks."
This last substitution signifies the scale over which people are aware
of their surroundings. Essentially this is an evolutionary, rather
than a theistic or geneological, view of the organization of the
Universe. In common with almost all creation myths, darkness precedes
light.

Gradually the early Greek philosopher-physicists replaced
earlier creation myths with speculative attempts at rational
explanations for the structure they knew. lonians, typified by Thales
of Miletus (about the sixth century B.C.), began this process by
claiming that all natural things came into being from other natural
things. The substance persists but its qualities change - a sort of
conservation law - according to Aristotle's later description
(Metaphysics). For Thales, water was the primitive element and he may
have thought (though this is uncertain) that the Earth originated from
water and continued to float on it. Suggestions (Kirk et al., 1983)
for Thales's choice of water range from the influence of earlier
Egyptian and Babylonian myths to the observation that corpses dry out.

Anaximander favored fire. Writing about the same time as
Thales, he states (in the description of Hippolytus, the Roman
theologian; Kirk et aL, p. 135): "The heavenly bodies came into being
as a circle of fire separated off from the fire in the world, and
enclosed by air. There are breathing-holes, certain pipe-like
passages, at which the heavenly bodies show themselves; accordingly
eclipses occur when the breathing holes are blocked up." Thus
cosmology continued its tradition of being often in error, but never
in doubt.

More modem views emerged a century later as Leucippus and
Democritus began to develop the implications of their atomic
ideas. Since Leucippus thought there were an infinite number of atoms,
it followed that there could be an infinite number of worlds.

The worlds come into being as follows: many bodies of all sorts of
shapes move "by abseission from the infinite" into a great void; they
come together there and produce a single whirl, in which, colliding
with one another and revolving in all manner of ways, they begin to
separate apart, like to like. But when their multitude prevents them
from rotating any longer in equilibrium, those that are fine go out
towards the surrounding void, as if sifted, while the rest "abide
together" and, becoming entangled, unite their motions and make a
first spherical structure.

So far, this sounds a bit like gravitational clustering, but then he
continues more fancifully:

This structure stands apart like a "membrane" which contains in itself
all kinds of bodies; and as they whirl around owing to the resistance
of the middle, the surrounding membrane becomes thin, while contiguous
atoms keep flowing together owing to contact with the whirl. So the
earth came into being, the atoms that had been borne to the middle
abiding together there. Again the containing membrane is itself
increased, owing to the attraction of bodies outside; as it moves
around in the whirl it takes in anything it touches. Some of these
bodies that get entangled form a structure that is at first moist and
muddy, but as they revolve with the whirl of the whole they dry out
and then ignite to form the substance of the heavenly bodies.
(Leucippus' theory described by Diogenes Laertius, a Roman biographer
of the third century A.D., in Kirk et al., p.4 17)

Thus the Universe is structured by natural, but ad hoc and
unexplained causes. That the atomists thought in terms of
short-range, rather than long-range, causes is suggested by
Simplicius, the Roman Neoplatonist commentator describing their views
in the sixth century A.D., a thousand years later: "...these atoms
move in the infinite void, separate one from the other and differing
in shapes, sizes, position and arrangement; overtaking each other they
collide, and some are shaken away in any chance direction, while
others, becoming intertwined one with another according to the
congruity of their shapes, sizes, positions and arrangements, stay
together and so effect the coming into being of compound bodies" (Kirk
et al., p. 426).

This atomic clustering theory coexisted with the more
continuum view of Dio- genes of Appolonia. Since Thales and
Anaximander had suggested water and fire, it was left to Diogenes to
posit air as the fundamental element of cosmogony: ". . . the whole
was in motion, and became rare in some places and dense in others;
where the dense ran together centripetally it made the earth, and so
the rest by the same method, while the lightest parts took the upper
position and produced the sun" (Plutarch, second century A.D. in Kirk
et al., p. 445). Diogenes may also have been the first to suggest that
the Universe contains dark matter, an extrapolation from the fall of a
large meteorite in Aegospotami (467 B.c.).

Plato and Aristotle, founders of the Academy and the
Peripatetic schools, were next to dominate philosophy for a hundred
years, fall into decline, and then be revived by the Romans and
preserved by the Arabs until in the thirteenth century they became
supreme authorities for four hundred years of scholastic church
commentary. Alive, they stimulated science, but as dead authorities
they were stultifying. Both of them abandoned the atomistic view that
the Universe was infinite. Plato, in his "Timaeus" proposed that the
Universe is analogous to a single unique living being (in his world of
ideal forms), created by a good and ideal god as a model of
himself. He made it spherical because it had no need of protuberances
for sight, hearing, or motion. The Moon, Sun, and Planets were the
first living gods, created to move around the Earth in ideal circles
so they could define and preserve Time. Aristotle attempted to give
somewhat more physical explanations. For example, in his "On the
Heavens" the Earth is spherical because each of its parts has weight
until it reaches the center. When Earth formed, all its parts sought
the center, their natural place of rest. This convergence produces an
object similar on all sides: a sphere. Here is one of the first
physical arguments using local symmetry. Aristotle then goes on to
cite two observations in support of a spherical Earth: the convex
boundary of lunar eclipses and the changing positions of the stars as
seen from different countries. He mentions the result of
mathematicians who used this second method to determine the Earth's
circumference; they found about twice the modern value.

Aristotle's arguments for a finite world and a unique Earth
were less satisfactory. First he claims there are two simple types of
motion, straight and circular. All others are compounds of
these. Simple bodies should follow simple motions. Circular motion,
being complete, is prior to rectilinear motion and therefore simple
prior bodies like planets and stars should follow it. Next he says
that only finite bodies can move in a circle, and since we see the
heaven revolving in a circle it must be finite. Finally, the world is
unique. For if there were more than one, simple bodies like earth and
fire would tend to move toward or away from the center of their
world. But since these bodies have the same natures wherever they are,
they would all have to move to the same center, implying there could
only be one world around this center.

During the temporary three hundred year decline of the
Academic school following the death of Aristotle in 322 B.C., atomism
resurged. It was carried to new heights by the Epicurians, culminating
in its great exposition by the Roman poet Lucretius in the middle of
the first century B.C.

Epicurus argued for an infinite Universe filled with an
infinite number of bodies (atoms). "For if the void were infinite but
the bodies finite, the bodies would not remain anywhere but would be
travelling scattered all over the infinite void, for lack of the
bodies which support and marshal them by buffeting. And if the void
were finite, the infinite bodies would not have anywhere to be" (Long
& Sedley, 1987, p. 44). Moreover, an infinite number of atoms could
produce an infinite number of worlds where a world contained an earth,
celestial bodies, and all the observable phenomena. This was necessary
because if, as in the Epicurian philosophy, structure results by
chance and not from teleology, the probability that our actual world
forms will increase dramatically if an infinite number of worlds are
possible. ". . . we must suppose that the worlds and every limited
compound which bears a close resemblance to the things we see, has
come into being from the infinite: all these things, the larger and
the smaller alike, have been separated off from it as a result of
individual entanglements. And all disintegrate again, some faster some
slower, and through differing kinds of courses" (Long & Sedley,
p. 57).

Rival Stoic philosophers who believed in a fiery continuum and
a finite world surrounded by an infinite void scoffed at the atheistic
Epicurian view. Indeed, Cicero, in his dialogs of the first century
B.C., which translated the Greek views into Latin, has his Stoic
spokesman say:

Does it not deserve amazement on my part that there should be anyone
who can persuade himself that certain solid and invisible bodies
travel through the force of their own weight and that by an accidental
combination of those bodies a world of the utmost splendour and beauty
is created? I do not see why the person who supposes this can happen
does not also believe it possible that if countless exemplars of the
twenty-one letters, in gold or any other material you like, were
thrown into a container then shaken out onto the ground, they might
form a readable copy of the Annals of Ennius. I'm not sure that luck
could manage this even to the extent of a single line! (Long &
Sedley, p. 328)

This may have been one of the earliest probability arguments about the
formation of world structure. Related arguments dominate much of the
present discussion on the subject, as we shall see.

But it was Lucretius, writing about the same time as Cicero,
who came closest to some ideas of modern cosmogony. The atoms of the
Universe are constantly in motion. Those whose shapes interweave and
stick together form dense aggregates like stone or iron. The less
dense atoms recoil and rebound (we would call it elastic scattering)
over great distances to provide the thin air; some atoms do not join
at all but wander everlastingly through the void. To create collisions
that produce structure, Lucretius introduces the idea of swerve:

On this topic, another thing I want you to know is this. When bodies
are being borne by their own weight straight down through the void, at
quite uncertain times and places they veer a little from their course,
just enough to be called a change of motion. If they did not have this
tendency to swerve, everything would be falling downward like
raindrops through the depths of the void, and collisions and impacts
among the primary bodies would not have arisen, with the result that
nature would never have created anything. (Long & Sedley, p. 49)

He almost seems to be portraying gravitational deflection. But then
Lucretius gets muddled up by the idea that everything must move at the
same speed through the unresisting void, independent of its
weight. Swerve must occur by just a minimum amount, so it cannot be
seen. And so the idea dissolves into an ad hoc hypothesis to save the
scheme. Nevertheless, his scheme gave rise to a statistical argument
often used today: "For so many primary particles have for an infinity
of time past been propelled in manifold ways by impacts and by their
own weight, and have habitually travelled, combined in all possible
ways, and tried out everything that their union could create, that it
is not surprising if they have also fallen into arrangements, and
arrived at patterns of motion, like those repeatedly enacted by this
present world" (Long & Sedley, p. 59). This is his rejoinder to
Cicero's Stoic.

Lucretius' most remarkable reason for believing that the
Universe was indeed infinite was to consider the difference this would
make to the large-scale distribution of matter.

Besides, if the totality of room in the whole universe were enclosed
by a fixed frontier on all sides, and were finite, by now the whole
stock of matter would through its solid weight have accumulated from
everywhere all the way to the bottom, and nothing could happen beneath
the sky's canopy, nor indeed could the sky or sunlight exist at all,
since all matter would be lying in a heap, having been sinking since
infinite time past. But as it is the primary bodies are clearly never
allowed to come to rest, because there is no absolute bottom at which
they might be able to accumulate and take up residence. At all times
all things are going on in constant motion everywhere, and underneath
there is a supply of particles of matter which have been travelling
from infinity. (Long & Sedley, p. 45)

At last in this brief sketch of nearly two dozen cosmogonies
we have found one that contains the seeds both of a rational
explanation and of a result that we can put into modern terms. But
this seed fell on infertile soil, for it was to take more than
seventeen hundred years before Isaac Newton added the motive force of
gravitation.